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Two‐effective‐source method for the calculation of in‐air output at various source‐to‐detector distances in wedged fields
Author(s) -
Kim Siyong,
Liu Chihray,
Chen Cody,
Palta Jatinder R.
Publication year - 1999
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.598487
Subject(s) - wedge (geometry) , collimator , physics , detector , optics , photon , position (finance) , economics , finance
A simple algorithm was developed for calculation of the in‐air output at various source‐to‐detector distances (SDDs) on the central axis for wedged fields. In the algorithm we dealt independently with two effective sources, one for head scatter and the other for wedge scatter. Varian 2100C with 18 and 8 MV photon beams was used to examine this algorithm. The effective source position for head scatter for wedged fields was assumed to be the same as that for open fields, and the effective source position for wedge scatter was assumed to be a certain distance upstream from the physical location of the wedge. The shift of the effective source for wedge scatter, w , was found to be independent of field size. Moreover, we observed no systematic dependency of w on wedge angle or beam energy. One value, w = 5.5 cm, provided less than 1% difference in in‐air outputs through the whole experimental range, i.e., 6 × 6 to 20 × 20cm 2field size ( 15 × 20cm 2for 60° wedge), 15°–60° wedge angle, 80–130 cm SDD, and both 18 and 8 MV photon beams. This algorithm can handle the case in which use of a tertiary collimator with an external wedge makes the field size for the determination of wedge scatter different from that for head scatter. In this case, without the two‐effective‐source method, the maximum of 4.7% and 2.6% difference can be given by the inverse square method and one‐effective‐source method in a 45° wedged field with 18 MV. Differences can be larger for thicker wedges. Enhanced dynamic wedge (EDW) fields were also examined. It was found that no second effective source is required for EDW fields.